/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.function.server;

import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.function.Supplier;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import reactor.core.publisher.Mono;

import org.springframework.core.io.Resource;
import org.springframework.http.codec.HttpMessageWriter;
import org.springframework.http.server.reactive.HttpHandler;
import org.springframework.util.Assert;
import org.springframework.web.reactive.result.view.ViewResolver;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebHandler;
import org.springframework.web.server.adapter.WebHttpHandlerBuilder;

/**
 * <strong>Central entry point to Spring's functional web framework.</strong>
 * Exposes routing functionality, such as to
 * {@linkplain #route(RequestPredicate, HandlerFunction) create} a {@code RouterFunction}
 * given a {@code RequestPredicate} and {@code HandlerFunction}, and to do further
 * {@linkplain #nest(RequestPredicate, RouterFunction) subrouting} on an existing routing
 * function.
 *
 * <p>Additionally, this class can {@linkplain #toHttpHandler(RouterFunction) transform} a
 * {@code RouterFunction} into an {@code HttpHandler}, which can be run in Servlet 3.1+,
 * Reactor, or Undertow.
 *
 * @author Arjen Poutsma
 * @since 5.0
 */
public abstract class RouterFunctions {

    private static final Log logger = LogFactory.getLog(RouterFunctions.class);

    /**
     * Name of the {@link ServerWebExchange} attribute that contains the {@link ServerRequest}.
     */
    public static final String REQUEST_ATTRIBUTE = RouterFunctions.class.getName() + ".request";

    /**
     * Name of the {@link ServerWebExchange} attribute that contains the URI
     * templates map, mapping variable names to values.
     */
    public static final String URI_TEMPLATE_VARIABLES_ATTRIBUTE =
            RouterFunctions.class.getName() + ".uriTemplateVariables";

    private static final HandlerFunction<ServerResponse> NOT_FOUND_HANDLER = request -> ServerResponse.notFound().build();


    /**
     * Route to the given handler function if the given request predicate applies.
     * <p>For instance, the following example routes GET requests for "/user" to the
     * {@code listUsers} method in {@code userController}:
     * <pre class="code">
     * RouterFunction&lt;ServerResponse&gt; route =
     *     RouterFunctions.route(RequestPredicates.GET("/user"), userController::listUsers);
     * </pre>
     *
     * @param predicate       the predicate to test
     * @param handlerFunction the handler function to route to if the predicate applies
     * @param <T>             the type of response returned by the handler function
     * @return a router function that routes to {@code handlerFunction} if
     * {@code predicate} evaluates to {@code true}
     * @see RequestPredicates
     */
    public static <T extends ServerResponse> RouterFunction<T> route(RequestPredicate predicate,
                                                                     HandlerFunction<T> handlerFunction) {

        Assert.notNull(predicate, "'predicate' must not be null");
        Assert.notNull(handlerFunction, "'handlerFunction' must not be null");

        return new DefaultRouterFunction<>(predicate, handlerFunction);
    }

    /**
     * Route to the given router function if the given request predicate applies. This method can be
     * used to create <strong>nested routes</strong>, where a group of routes share a common path
     * (prefix), header, or other request predicate.
     * <p>For instance, the following example first creates a composed route that resolves to
     * {@code listUsers} for a GET, and {@code createUser} for a POST. This composed route then gets
     * nested with a "/user" path predicate, so that GET requests for "/user" will list users,
     * and POST request for "/user" will create a new user.
     * <pre class="code">
     * RouterFunction&lt;ServerResponse&gt; userRoutes =
     *   RouterFunctions.route(RequestPredicates.method(HttpMethod.GET), this::listUsers)
     *     .andRoute(RequestPredicates.method(HttpMethod.POST), this::createUser);
     *
     * RouterFunction&lt;ServerResponse&gt; nestedRoute =
     *   RouterFunctions.nest(RequestPredicates.path("/user"),userRoutes);
     * </pre>
     *
     * @param predicate      the predicate to test
     * @param routerFunction the nested router function to delegate to if the predicate applies
     * @param <T>            the type of response returned by the handler function
     * @return a router function that routes to {@code routerFunction} if
     * {@code predicate} evaluates to {@code true}
     * @see RequestPredicates
     */
    public static <T extends ServerResponse> RouterFunction<T> nest(RequestPredicate predicate,
                                                                    RouterFunction<T> routerFunction) {

        Assert.notNull(predicate, "'predicate' must not be null");
        Assert.notNull(routerFunction, "'routerFunction' must not be null");

        return new DefaultNestedRouterFunction<>(predicate, routerFunction);
    }

    /**
     * Route requests that match the given pattern to resources relative to the given root location.
     * For instance
     * <pre class="code">
     * Resource location = new FileSystemResource("public-resources/");
     * RoutingFunction&lt;ServerResponse&gt; resources = RouterFunctions.resources("/resources/**", location);
     * </pre>
     *
     * @param pattern  the pattern to match
     * @param location the location directory relative to which resources should be resolved
     * @return a router function that routes to resources
     */
    public static RouterFunction<ServerResponse> resources(String pattern, Resource location) {
        Assert.hasLength(pattern, "'pattern' must not be empty");
        Assert.notNull(location, "'location' must not be null");
        return resources(new PathResourceLookupFunction(pattern, location));
    }

    /**
     * Route to resources using the provided lookup function. If the lookup function provides a
     * {@link Resource} for the given request, it will be it will be exposed using a
     * {@link HandlerFunction} that handles GET, HEAD, and OPTIONS requests.
     *
     * @param lookupFunction the function to provide a {@link Resource} given the {@link ServerRequest}
     * @return a router function that routes to resources
     */
    public static RouterFunction<ServerResponse> resources(Function<ServerRequest, Mono<Resource>> lookupFunction) {
        Assert.notNull(lookupFunction, "'lookupFunction' must not be null");
        return new ResourcesRouterFunction(lookupFunction);
    }

    /**
     * Convert the given {@linkplain RouterFunction router function} into a {@link HttpHandler}.
     * This conversion uses {@linkplain HandlerStrategies#builder() default strategies}.
     * <p>The returned handler can be adapted to run in
     * <ul>
     * <li>Servlet 3.1+ using the
     * {@link org.springframework.http.server.reactive.ServletHttpHandlerAdapter},</li>
     * <li>Reactor using the
     * {@link org.springframework.http.server.reactive.ReactorHttpHandlerAdapter},</li>
     * <li>Undertow using the
     * {@link org.springframework.http.server.reactive.UndertowHttpHandlerAdapter}.</li>
     * </ul>
     * <p>Note that {@code HttpWebHandlerAdapter} also implements {@link WebHandler}, allowing
     * for additional filter and exception handler registration through
     * {@link WebHttpHandlerBuilder}.
     *
     * @param routerFunction the router function to convert
     * @return an http handler that handles HTTP request using the given router function
     */
    public static HttpHandler toHttpHandler(RouterFunction<?> routerFunction) {
        return toHttpHandler(routerFunction, HandlerStrategies.withDefaults());
    }

    /**
     * Convert the given {@linkplain RouterFunction router function} into a {@link HttpHandler},
     * using the given strategies.
     * <p>The returned {@code HttpHandler} can be adapted to run in
     * <ul>
     * <li>Servlet 3.1+ using the
     * {@link org.springframework.http.server.reactive.ServletHttpHandlerAdapter},</li>
     * <li>Reactor using the
     * {@link org.springframework.http.server.reactive.ReactorHttpHandlerAdapter},</li>
     * <li>Undertow using the
     * {@link org.springframework.http.server.reactive.UndertowHttpHandlerAdapter}.</li>
     * </ul>
     *
     * @param routerFunction the router function to convert
     * @param strategies     the strategies to use
     * @return an http handler that handles HTTP request using the given router function
     */
    public static HttpHandler toHttpHandler(RouterFunction<?> routerFunction, HandlerStrategies strategies) {
        Assert.notNull(routerFunction, "RouterFunction must not be null");
        Assert.notNull(strategies, "HandlerStrategies must not be null");

        WebHandler webHandler = toWebHandler(routerFunction, strategies);
        return WebHttpHandlerBuilder.webHandler(webHandler)
                .filters(filters -> filters.addAll(strategies.webFilters()))
                .exceptionHandlers(handlers -> handlers.addAll(strategies.exceptionHandlers()))
                .localeContextResolver(strategies.localeContextResolver())
                .build();
    }

    /**
     * Convert the given {@linkplain RouterFunction router function} into a {@link WebHandler}.
     * This conversion uses {@linkplain HandlerStrategies#builder() default strategies}.
     *
     * @param routerFunction the router function to convert
     * @return a web handler that handles web request using the given router function
     */
    public static WebHandler toWebHandler(RouterFunction<?> routerFunction) {
        return toWebHandler(routerFunction, HandlerStrategies.withDefaults());
    }

    /**
     * Convert the given {@linkplain RouterFunction router function} into a {@link WebHandler},
     * using the given strategies.
     *
     * @param routerFunction the router function to convert
     * @param strategies     the strategies to use
     * @return a web handler that handles web request using the given router function
     */
    public static WebHandler toWebHandler(RouterFunction<?> routerFunction, HandlerStrategies strategies) {
        Assert.notNull(routerFunction, "RouterFunction must not be null");
        Assert.notNull(strategies, "HandlerStrategies must not be null");

        return exchange -> {
            ServerRequest request = new DefaultServerRequest(exchange, strategies.messageReaders());
            addAttributes(exchange, request);
            return routerFunction.route(request)
                    .defaultIfEmpty(notFound())
                    .flatMap(handlerFunction -> wrapException(() -> handlerFunction.handle(request)))
                    .flatMap(response -> wrapException(() -> response.writeTo(exchange,
                            new HandlerStrategiesResponseContext(strategies))));
        };
    }

    private static <T> Mono<T> wrapException(Supplier<Mono<T>> supplier) {
        try {
            return supplier.get();
        } catch (Throwable t) {
            return Mono.error(t);
        }
    }

    private static void addAttributes(ServerWebExchange exchange, ServerRequest request) {
        Map<String, Object> attributes = exchange.getAttributes();
        attributes.put(REQUEST_ATTRIBUTE, request);
    }

    @SuppressWarnings("unchecked")
    private static <T extends ServerResponse> HandlerFunction<T> notFound() {
        return (HandlerFunction<T>) NOT_FOUND_HANDLER;
    }

    @SuppressWarnings("unchecked")
    static <T extends ServerResponse> HandlerFunction<T> cast(HandlerFunction<?> handlerFunction) {
        return (HandlerFunction<T>) handlerFunction;
    }


    /**
     * Receives notifications from the logical structure of router functions.
     */
    public interface Visitor {

        /**
         * Receive notification of the beginning of a nested router function.
         *
         * @param predicate the predicate that applies to the nested router functions
         * @see RouterFunctions#nest(RequestPredicate, RouterFunction)
         */
        void startNested(RequestPredicate predicate);

        /**
         * Receive notification of the end of a nested router function.
         *
         * @param predicate the predicate that applies to the nested router functions
         * @see RouterFunctions#nest(RequestPredicate, RouterFunction)
         */
        void endNested(RequestPredicate predicate);

        /**
         * Receive notification of a standard predicated route to a handler function.
         *
         * @param predicate       the predicate that applies to the handler function
         * @param handlerFunction the handler function.
         * @see RouterFunctions#route(RequestPredicate, HandlerFunction)
         */
        void route(RequestPredicate predicate, HandlerFunction<?> handlerFunction);

        /**
         * Receive notification of a resource router function.
         *
         * @param lookupFunction the lookup function for the resources
         * @see RouterFunctions#resources(Function)
         */
        void resources(Function<ServerRequest, Mono<Resource>> lookupFunction);

        /**
         * Receive notification of an unknown router function. This method is called for router
         * functions that were not created via the various {@link RouterFunctions} methods.
         *
         * @param routerFunction the router function
         */
        void unknown(RouterFunction<?> routerFunction);
    }


    private static abstract class AbstractRouterFunction<T extends ServerResponse> implements RouterFunction<T> {

        @Override
        public String toString() {
            ToStringVisitor visitor = new ToStringVisitor();
            accept(visitor);
            return visitor.toString();
        }
    }

    final static class SameComposedRouterFunction<T extends ServerResponse> extends AbstractRouterFunction<T> {

        private final RouterFunction<T> first;

        private final RouterFunction<T> second;

        public SameComposedRouterFunction(RouterFunction<T> first, RouterFunction<T> second) {
            this.first = first;
            this.second = second;
        }

        @Override
        public Mono<HandlerFunction<T>> route(ServerRequest request) {
            return this.first.route(request)
                    .switchIfEmpty(Mono.defer(() -> this.second.route(request)));
        }

        @Override
        public void accept(Visitor visitor) {
            this.first.accept(visitor);
            this.second.accept(visitor);
        }
    }

    final static class DifferentComposedRouterFunction extends AbstractRouterFunction<ServerResponse> {

        private final RouterFunction<?> first;

        private final RouterFunction<?> second;

        public DifferentComposedRouterFunction(RouterFunction<?> first, RouterFunction<?> second) {
            this.first = first;
            this.second = second;
        }

        @Override
        public Mono<HandlerFunction<ServerResponse>> route(ServerRequest request) {
            return this.first.route(request)
                    .map(RouterFunctions::cast)
                    .switchIfEmpty(Mono.defer(() -> this.second.route(request).map(RouterFunctions::cast)));
        }

        @Override
        public void accept(Visitor visitor) {
            this.first.accept(visitor);
            this.second.accept(visitor);
        }

    }

    final static class FilteredRouterFunction<T extends ServerResponse, S extends ServerResponse>
            implements RouterFunction<S> {

        private final RouterFunction<T> routerFunction;

        private final HandlerFilterFunction<T, S> filterFunction;

        public FilteredRouterFunction(
                RouterFunction<T> routerFunction,
                HandlerFilterFunction<T, S> filterFunction) {
            this.routerFunction = routerFunction;
            this.filterFunction = filterFunction;
        }

        @Override
        public Mono<HandlerFunction<S>> route(ServerRequest request) {
            return this.routerFunction.route(request).map(this.filterFunction::apply);
        }

        @Override
        public void accept(Visitor visitor) {
            this.routerFunction.accept(visitor);
        }

    }

    private static final class DefaultRouterFunction<T extends ServerResponse>
            extends AbstractRouterFunction<T> {

        private final RequestPredicate predicate;

        private final HandlerFunction<T> handlerFunction;

        public DefaultRouterFunction(RequestPredicate predicate,
                                     HandlerFunction<T> handlerFunction) {
            this.predicate = predicate;
            this.handlerFunction = handlerFunction;
        }

        @Override
        public Mono<HandlerFunction<T>> route(ServerRequest request) {
            if (this.predicate.test(request)) {
                if (logger.isDebugEnabled()) {
                    logger.debug(String.format("Predicate \"%s\" matches against \"%s\"",
                            this.predicate, request));
                }
                return Mono.just(this.handlerFunction);
            } else {
                return Mono.empty();
            }
        }

        @Override
        public void accept(Visitor visitor) {
            visitor.route(this.predicate, this.handlerFunction);
        }

    }

    private static final class DefaultNestedRouterFunction<T extends ServerResponse>
            extends AbstractRouterFunction<T> {

        private final RequestPredicate predicate;

        private final RouterFunction<T> routerFunction;

        public DefaultNestedRouterFunction(RequestPredicate predicate,
                                           RouterFunction<T> routerFunction) {
            this.predicate = predicate;
            this.routerFunction = routerFunction;
        }

        @Override
        public Mono<HandlerFunction<T>> route(ServerRequest serverRequest) {
            return this.predicate.nest(serverRequest)
                    .map(nestedRequest -> {
                                if (logger.isDebugEnabled()) {
                                    logger.debug(
                                            String.format(
                                                    "Nested predicate \"%s\" matches against \"%s\"",
                                                    this.predicate, serverRequest));
                                }
                                return this.routerFunction.route(nestedRequest);
                            }
                    )
                    .orElseGet(Mono::empty);
        }

        @Override
        public void accept(Visitor visitor) {
            visitor.startNested(this.predicate);
            this.routerFunction.accept(visitor);
            visitor.endNested(this.predicate);
        }

    }

    private static class ResourcesRouterFunction extends AbstractRouterFunction<ServerResponse> {

        private final Function<ServerRequest, Mono<Resource>> lookupFunction;

        public ResourcesRouterFunction(Function<ServerRequest, Mono<Resource>> lookupFunction) {
            this.lookupFunction = lookupFunction;
        }

        @Override
        public Mono<HandlerFunction<ServerResponse>> route(ServerRequest request) {
            return this.lookupFunction.apply(request).map(ResourceHandlerFunction::new);
        }

        @Override
        public void accept(Visitor visitor) {
            visitor.resources(this.lookupFunction);
        }
    }

    private static class HandlerStrategiesResponseContext implements ServerResponse.Context {

        private final HandlerStrategies strategies;

        public HandlerStrategiesResponseContext(HandlerStrategies strategies) {
            this.strategies = strategies;
        }

        @Override
        public List<HttpMessageWriter<?>> messageWriters() {
            return this.strategies.messageWriters();
        }

        @Override
        public List<ViewResolver> viewResolvers() {
            return this.strategies.viewResolvers();
        }
    }

}
